1. Field of the Invention
The present invention relates to a wire-cut, electric discharge machining method which ensures automatic insertion of the tip of a wire electrode into the machining start hole even if there is scale or the like developed on the inside of a machining start hole or there is a relief on the back of a workpiece.
2. Description of the Prior Art
In a wire-cut, electric discharge machine of the type in which a pulse-like voltage is applied across a linear electrode, such as a wire or the like, and a workpiece to produce therebetween a discharge for machining the workpiece into a desired configuration, the machining is usually started from a machining start hole made in the workpiece for the insertion thereinto of the wire.
Ordinarily workpieces are mostly formed of iron or like material and heat treated. Since the machining start hole is usually made prior to the heat treatment, scale or the like is developed by the heat treatment on the inside of the machining start hole. Accordingly in order to provide a good surface roughness on the inner surface of the machining start hole, the scale or the like must be removed, for example, by means of machining with reamer, but the removing machining is difficult and time-consuming because the machining start hole is small and the scale sticks to the inside of the machining start hole.
In recent years there has often been employed a wire-cut, electric discharge machine such as shown in FIG. 1 which uses feed rollers 6 revolving with a wire 4 gripped therebetween to raise it and a wire guide nozzle 5 guiding the wire 4 to move it up, automatically inserting its tip into a machining start hole 3 made in a workpiece 1. In the case where there is a relief 2 on the back of the workpiece 1 as shown in FIG. 1, however, the tip of the wire 4 may not sometimes be inserted into the machining start hole 3; therefore, there is a strong demand for improvement of the prior art in this respect. In FIG. 1, reference numeral 7 indicates a V-groove guide; 8 designates a conducting pin; 9 and 10 identify guide rollers; 11 denotes a brake; and 12 represents a wire reel.